#app4 {}

@keyframes change {
    0% {
        background: black;
    }

    100% {
        background: red;
    }
}

#app4 .circle {
    border: 1px solid black;
    width: 15vw;
    height: 15vw;
    border-radius: 50%;
    margin: 5vw 0 0 5vw;
}

/*alternate 轮流的*/
#app4 .circle.active {
    animation: change 3s linear alternate infinite;
}